Acid functional fluoropolymer membranes and method of manufacture

ABSTRACT

Methods are provided to make acid functional fluoropolymers by: a) dehydrofluorinating a starting fluoropolymer with a dehydrofluorinating agent to form an unsaturated fluoropolymer; b) adding an acidifiable nucleophilic functionalizing agent to a double bond of the unsaturated fluoropolymer; and c) acidifying the added acidifiable function. Acid functional fluoropolymers and ion conducting membranes thereof are also provided, including acid functional fluoropolymer having pendent groups according to the formula: —X—Ar—A n , wherein X is selected from O, S or NR, where R is selected from H and C1-C30 alkyl or aryl, which are optionally substituted, wherein Ar is a C6-C30 aromatic group, which is optionally substituted, wherein A is an acidic function or salt thereof, wherein a can be independently chosen to be 1, 2 or 3.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. Ser. No. 09/464,337,filed Dec. 15, 1999, now allowed, the disclosure of which is hereinincorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to novel methods of synthesizing acidfunctional fluoropolymers by dehydrofluorination and nucleophilicaddition of an acidifiable group, followed by acidification of theacidifiable group to obtain acid functionality. This invention alsorelates to the polymers and membranes made or modified according to suchmethods, particularly for use as ion conducting membranes.

BACKGROUND OF THE INVENTION

[0003] Electrochemical devices, including proton exchange membrane fuelcells, electrolyzers, chlor-alkali separation membranes, and the like,are typically constructed from a unit referred to as a membraneelectrode assembly (MEA). Such MEA's comprise one or more electrodeportions, which include a catalytic electrode material such as Pt or Pd,in contact with an ion conductive membrane. Ion conductive membranes(ICMs) are used in electrochemical cells as solid electrolytes. In atypical electrochemical cell, an ICM is in contact with cathode andanode electrodes, and transports ions such as protons that are formed atthe anode to the cathode, allowing a current of electrons to flow in anexternal circuit connecting the electrodes.

[0004] In a typical hydrogen/oxygen fuel cell, the ions to be conductedby the membrane are protons. Importantly, ICMs do not conductelectrons/electricity, since this would render the fuel cell useless,and they must be essentially impermeable to fuel gasses, such ashydrogen and oxygen. Any leakage of the gasses employed in the reactionacross the MEA results in waste of the reactants and inefficiency of thecell. For that reason, the ion exchange membrane must have low or nopermeability to the gasses employed in the reaction.

[0005] ICMs also find use in chlor-alkali cells wherein brine mixturesare separated to form chlorine gas and sodium hydroxide. The membraneselectively transports sodium ions while rejecting chloride ions. Suchmembranes may also be useful in batteries and electrochemical storagecells, particularly membranes that transport lithium ions. ICMs also canbe useful for applications such as diffusion dialysis, electrodialysis,and pervaporization and vapor permeation separations. While most ICMstransport cations or protons, membranes that are transportive to anionssuch as OH are known and commercially available.

[0006] Commercially-available ICMs are not entirely satisfactory inmeeting the performance demands of fuel cells. For example, Nafion™membranes (DuPont Chemicals, Inc., Wilmington, Del.), which areperfluorocarbon materials having pendent sulfonate groups, areconsidered expensive and structurally weak when wet. Nafion membranesare not generally available at thicknesses of less than 50 μm. WhileNafion membranes with lower equivalent weight can be used to obtainlower ionic resistance, lower equivalent weight membranes arestructurally weaker and thus require reinforcement.

[0007] The search for new acid-functional fluoropolymers has beenimpeded by the difficulty inherent in copolymerizing acid-functionalfluoromonomers with tetrafluoroethylene or other suitable perfluorocomonomers.

[0008] U.S. Pat. No. 4,894,410 and U.S. Pat. No. 4,956,419 (3M) disclosethe manufacture of fluoropolymer membranes having various functionalgroups appended through thio linkages.

[0009] U.S. Pat. No. 5,395,886 (Dow Corning) discloses a method ofmodifying partially-fluorinated hydrocarbon polymers to provide latentreactive substituents and polymers crosslinked by means of thosesubstituents. The latent reactive substituents are appended bynucleophilic addition subsequent or concurrent to dehydrofluorination ofthe polymer. The reference does not disclose a polymer membranesufficiently substituted with acidic functions to function as an ionconducting membrane.

[0010] U.S. Pat. No. 5,656,386 (Paul Scherrer Institut) disclosesfluoropolymer membranes having various functional groups appended by aradiation grafting method.

SUMMARY OF THE INVENTION

[0011] Briefly, the present invention provides a method of making anacid functional fluoropolymer by: a) dehydrofluorinating a startingfluoropolymer with a dehydrofluorinating agent to form an unsaturatedfluoropolymer; b) adding an acidifiable nucleophilic functionalizingagent to a double bond of the unsaturated fluoropolymer; and c)acidifying the added acidifiable function.

[0012] In another aspect, the present invention provides acid functionalfluorocarbon membranes for use as ion conducting membranes inelectrochemical cells.

[0013] In another aspect, the present invention provides a method ofmaking an ion conducting membrane (ICM) by: a) dehydrofluorinating astarting fluoropolymer with a dehydrofluorinating agent to form anunsaturated fluoropolymer; b) adding an acidifiable nucleophilicfunctionalizing agent to a double bond of the unsaturated fluoropolymerto form a fluoropolymer bearing an acidifiable function; c) forming thefluoropolymer bearing an acidifiable function into a membrane; and d)acidifying said acidifiable function to form an ICM.

[0014] In another aspect, the present invention provides acid functionalfluoropolymers having pendent groups according to the formula:—X—Ar—A_(n), wherein X is selected from O, S or NR, where R is selectedfrom H and C1-C30 alkyl or aryl, which are optionally substituted,wherein Ar is a C6-C30 aromatic group, which is optionally substituted,wherein A is an acidic function or salt thereof, wherein a can beindependently chosen to be 1, 2 or 3; and wherein said acid functionalfluoropolymer is sufficiently acidified as to meet a condition selectedfrom: a) the equivalent weight of the polymer is 5000 or less; and b)the proton conductivity of the polymer at 25° C. is 0.01 Siemens percentimeter (S/cm) or higher. In addition, ion conducting membranes ofsuch acid functional fluoropolymers are provided.

[0015] What has not been described in the art, and is provided by thepresent invention, is a method of providing acid functionalizedfluoropolymer materials usable as ion conducting membranes, such asthose used in electrolytic cells.

[0016] In this application,

[0017] “acidifiable” group, function or agent means either a) anacid-receiving group which is readily capable of substitution with anacid function, preferably by exposure to an acid, such as an aromaticgroup which may be acidified by treatment with sulfuric acid, or b) aproto-acid function which is capable of facile conversion to an acid,preferably by hydrolysis, such as a sulfonyl halide, but preferably a);

[0018] “equivalent weight” means the mass of an acidic material thatcontains one mole of acid functional groups; and

[0019] when used without reference to a particular substituent,“substituted” means, for a chemical species, substituted by conventionalsubstituents which do not interfere with the desired product or process,e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br,I), cyano, nitro, etc.

[0020] It is an advantage of the present invention to providefluoropolymer ion conducting membranes for use in an electrolytic cell.It is a further advantage to provide a simple synthetic route to suchmembranes.

BRIEF DESCRIPTION OF THE DRAWING

[0021]FIG. 1 is a chart of weight loss vs. time under oxidativeconditions for one membrane of the present invention (Trace A) and twocomparative membranes (Traces B and C).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] The present invention provides a method of making an acidfunctional fluoropolymer by: a) dehydrofluorinating a startingfluoropolymer with a dehydrofluorinating agent to form an unsaturatedfluoropolymer; b) adding an acidifiable nucleophilic functionalizingagent to a double bond of the unsaturated fluoropolymer; and c)acidifying the added acidifiable function.

[0023] Starting Fluoropolymer

[0024] The starting polymer may be any fluoropolymer having hydrogen andfluorine substituents on adjacent carbons, where the hydrogen andfluorine may be abstracted to form a double bond. These >CH—CF< moietiespreferably occur in the polymer backbone but may also occur in pendantgroups or branches. Preferably the starting fluoropolymer is between 50%and 95% fluorinated, i.e. between 50% and 95% of C—H bonds are replacedwith C—F bonds. More preferably the starting fluoropolymer is between50% and 80% fluorinated and most preferably 65-75%. The startingfluoropolymer may be additionally substituted but is preferably notadditionally substituted.

[0025] Preferably, the starting polymer is a polymer or copolymer ofvinylidene fluoride (1,1-difluoroethene). More preferably, the startingpolymer is a copolymer of vinylidene fluoride and hexafluoropropene,such as Fluorel™ (Dyneon Corp., Oakdale, Minn.), THV™ (Dyneon Corp.,Oakdale, Minn.), or Viton™ (DuPont de Nemours and Co., Wilmington,Del.).

[0026] The starting polymer is preferably in the form of a membranewhich is advantageously impervious or substantially impervious topassage of gasses, particularly reactant gasses used in electrochemicalcells, such as air, oxygen, hydrogen, chlorine, and the like. Themembrane is preferably 200 μm or less in thickness, more preferably 50μm or less and most preferably 25 μm or less.

[0027] Dehydrofluorinating and Functionalizing Agents

[0028] The dehydrofluorinating agent may be any species capable ofabstracting a hydrogen from the starting fluoropolymer, including strongbases. The nucleophilic functionalizing agent may be any species capableof addition to a double bond of the fluoropolymer, in keeping with themethod of the present invention. Preferably the dehydrofluorinatingagent and the nucleophilic functionalizing agent are one and the same.

[0029] Preferably the functionalizing agent comprises a nucleophilicgroup such as an anionic nitrogen-, oxygen- and/or sulfur-containinggroup, preferably an oxide or sulfide group, and most preferably oxide.Any suitable counter ion may be used.

[0030] The functionalizing agent comprises an acidifiable group, whichmay be an acid-receiving group which is readily capable of substitutionwith an acid function, or a proto-acid function which is readily capableof conversion to an acid, but is preferably an acid-receiving group.Acidifiable groups include those containing aromatic functions,including heteroaromatic functions, preferably C6-C30 aromatic groupsand most preferably phenyl. Acidifiable aromatic groups may haveelectron donating substituents which aid in later addition of an acidgroup. Proto-acid groups include groups readily hydrolyzable to formacid functional groups such as esters, anhydrides or acid halides,preferably sulfonylhalides and especially sulfonyl chloride. Theacidifiable group may be fluorinated.

[0031] Preferred acid-receiving functionalizing agents having theformula: —X—Ar, wherein X is selected from O, S or NR, where R isselected from H and C1-C30 alkyl or aryl, which are optionallysubstituted, and wherein Ar is a C6-C30 aromatic group, which isoptionally substituted. This agent may be advantageously substitutedwith electron donor groups. More preferably, the nucleophilicfunctionalizing agent is an aryloxide (ArO⁻). Most preferably, thenucleophilic functionalizing agent is phenoxide (PhO⁻).

[0032] Functionalizing Conditions

[0033] The starting fluoropolymer and functionalizing agent may bereacted by any suitable means. The reactants may be combined insolution. A base to absorb HF as formed during dehydrofluorination maybe advantageously employed, such as Li₂CO₃. The resulting productsolution may be decanted and the solvent removed to yield the productresin.

[0034] The steps of a) dehydrofluorinating the starting fluoropolymerand b) adding an acidifiable nucleophilic functionalizing agent to adouble bond of the unsaturated fluoropolymer can be carried outsequentially or simultaneously.

[0035] Fluoropolymers functionalized with acid-receiving groups may beacidified by any suitable means, including exposure to sulfuric,phosphoric or other acids which may bind covalently to the acidifiablegroup. Preferably, the functionalized polymer is immersed inconcentrated and/or fuming sulfuric acid for 24 hours or more at 25° C.or higher temperature.

[0036] The equivalent weight of an acidic material is the mass thatcontains one mole of acidic hydrogen. The equivalent weight of an ionconducting membrane is the number of acidic group equivalents in thepolymeric membrane divided by the weight of the polymer. Lower values ofequivalent weight generally correspond with increased ionicconductivity. The materials according to the present invention can beadvantageously made having an equivalent weight of 5000 or less, morepreferably 3000 or less, even more preferably 1600 or less, and mostpreferably 1250 or less while maintaining sufficient mechanical strengthand dimensional stability for use as an ICM.

[0037] Polymers with high proton conductivity are desirable for use inan ICM. High proton conductivity with low resistive loss is advantageousto support high current levels in fuel cell use. The materials accordingto the present invention can be made having an ionic conductivity of0.01 Siemens per centimeter (S/cm) or greater at room temperature (25°C.), more preferably 0.05 S/cm or greater, and most preferably 0.10 S/cmor greater. For lithium ion conducting membranes, ionic conductivity ispreferably 0.04 mS/cm or greater at room temperature, more preferably0.3 mS/cm or greater. The ionic conductivity is preferably the same orhigher in the operating temperature range of the electrochemical devicein which the ICM is used.

[0038] The fluoropolymer may be blended with a second polymer afterreaction with the functionalizing agent and preferably before anyacidification step. Any suitable second polymer may be used, butfluoropolymers suitable as starting fluoropolymers, as described above,are preferred. Blending may be accomplished by any suitable method.

[0039] The fluoropolymer may be crosslinked using any suitablecrosslinking agent or method, including free radical and nucleophilicprocesses. Preferred crosslinking agents include Bisphenol AF.Crosslinking may be carried out at any step of the functionalizingprocess. The fluoropolymer is preferably not crosslinked through thefunctionalizing agent. The fluoropolymer is preferably not crosslinkedthrough an acid group added according to the method of the presentinvention.

[0040] Each step may be performed batchwise or in continuous fashion.

[0041] Ion Conducting Membranes

[0042] The method of the present invention is especially useful in thefabrication of ion conducting membranes (ICM's). ICM's may be madeaccording to the present invention by acidification of a fluoropolymerwith acid functional groups including carboxyl, sulfonyl, and/orphosphonyl groups, preferably sulfonyl groups. Most preferably, asuitable fluoropolymer is functionalized with an aryloxide, mostpreferably phenoxide, and then sulfonated, as described above and in theExamples below.

[0043] The starting fluoropolymer may be formed into a membrane by anysuitable means, including casting, coating, pressing, extruding, and thelike, but most preferably coating. Membrane formation may be carried outprior to addition of the acidifiable function, after addition of theacidifiable function but prior to acidification, or after acidification.Preferably, the polymer is formed into a membrane afterfunctionalization but prior to acidification. Most preferably, theacidifiable function is added to the polymer in solution, the polymer isthen cast or coated to form a membrane, and then the membrane isacidified. Such membranes are hydrated or saturated with liquid tobecome ion conducting.

[0044] The resulting functionalized polymer membrane is advantageouslyimpervious or substantially impervious to passage of gasses,particularly reactant gasses used in electrochemical cells, such as air,oxygen, hydrogen, chlorine, and the like. The membrane is preferably 200μm or less in thickness, more preferably 50 μm or less and mostpreferably 25 μm or less. Advantageously, the membrane is essentiallynon-conductive to electricity.

[0045] ICM's advantageously have a low equivalent weight, a high ionicconductivity, high stability under oxidative conditions, sufficientmechanical strength, and stability under high temperature conditions.

[0046] Proton conducting membranes may be converted so as to conductother cations by any suitable method of ion exchange, such as soaking ina solution of the hydroxide of the cation to be introduced to themembrane.

[0047] ICM's of the present invention preferably demonstrate highoxidative stability, preferably retaining 90% or more of their originalweight after exposure to a 3% hydrogen peroxide solution containing 4ppm iron sulfate at 68° C. for seven days and more preferably retaining95% or more of their original weight.

[0048] This invention is useful in synthesizing functionalizedfluoropolymers, in particular acid functional fluoropolymer membranesfor use as ion conducting membranes in electrochemical cells such ashydrogen fuel cells or lithium ion batteries.

[0049] Objects and advantages of this invention are further illustratedby the following examples, but the particular materials and amountsthereof recited in these examples, as well as other conditions anddetails, should not be construed to unduly limit this invention.

EXAMPLES

[0050] Unless otherwise noted, all chemicals and reagents were obtainedor are available from Aldrich Chemical Co., Milwaukee, Wis.

Example 1

[0051] Functionalized fluoropolymer films according to the presentinvention having sulfonated phenoxy substituents were made and testedalong with comparative films, as described in Table I and the textfollowing.

[0052] Membranes Nos. 22C and 23C appearing in Table I are twocommercially available comparative membranes: Nafion™ 117 (DuPontChemicals, Wilmington, Del., available from ElectroChem, Inc., Woburn,Mass., and Aldrich Chemical Co., Inc., Milwaukee, Wis.) and Bio-Rex™ IonExchange Membrane AG 50W-18 (Bio-Rad, Hercules, Calif.).

[0053] RT indicates room temperature, i.e., about 25° C. TABLE IFP/phenoxide Blended with H₂SO₄ Equivalent Conductivity No. FP weightratio Second FP Crosslinker time/temp Weight (g/mol) (mS/cm)  1 FC214568.7/31.3 none none 1 day/RT 17.4(25° C.)  2 FC2145 68.7/31.3 none none2 days/RT 2154 26.8(25° C.)  3 FC2145 68.7/31.3 none none 4 days/RT  4FC2145 84.8/15.2 none none 3 days/RT 2800 1.0(25° C.)/1.2(80° C.)  5FC2145 84.8/15.2 none none 7 days/RT  6 FC2145 84.8/15.2 none   9.8% 7days/RT 17(25° C.)/28(80° C.)  7 FC2145 68.7/31.3 none none 5 days/RT656  8 FC2145 68.7/31.3 10% FC2145 none 5 days/RT  9 FC2145 68.7/31.3none  2% 5 days/RT 10 FC2145 68.7/31.3 none  4% 5 days/RT 11 FC214568.7/31.3 none 10% 5 days/RT 12 FC2145 68.7/31.3 10% FC2178 10% 3days/RT 850 13 FC2145 68.7/31.3 20% FC2145 10% 3 days/RT 1100 14 FC214568.7/31.3 none none 1 day/50° C. 843 15 FC2145 68.7/31.3 20% FC2178 10%3 days/50° C. 1070 65(25° C.)/94(82° C.) 16 FC2145 68.7/31.3 20% THV-20010% 1 day/50° C. 1545 17 FC2145 68.7/31.3 20% THV-200 none 1 day/50° C.18 FC2145 68.7/31.3 25% THV-200  2% 1 day/50° C. 19 FC2178 68.7/31.3none  2% 1 day/50° C. 710 20 FC2178 68.7/31.3 10% FC2178  2% 1 day/50°C. 748 21 FC2178 68.7/31.3 10% THV-200  2% 1 day/50° C. 1000 22C Nafion117 1100 83(25° C.) 23C Bio-Rex Ag50-W-18 300 4(25° C.)/8(80° C.)

[0054] Phenoxy-Substituted Fluoropolymers

[0055] Two fluoropolymer resins were used, Fluorel FC2145 and FluorelFC2178. For each membrane in Table I, a weighed amount of the indicatedfluoropolymer resin (FP) was dissolved in methyl ethyl ketone (MEK) at aconcentration of roughly 15% by weight. A volume of 1 M lithiumphenoxide solution in THF (Aldrich Chemical Co., Milwaukee, Wis.) wasadded, to provide the indicated weight ratio of fluoropolymer tophenoxide. Two equivalents (based on Li phenoxide) of Li₂CO₃ were added.The solution was heated to reflux and stirred by a mechanical stirrerfor 3-5 days. The solution was then left sitting at room temperatureovernight to allow the Li₂CO₃ to settle. The resulting yellow/brownsolution was then decanted and the solvent was removed under anaspirator and then a vacuum pump to yield a brown resin. Where blendingwith a second fluoropolymer is indicated in Table I, the resin wasredisolved in MEK, blended by stirring with the indicated amount (as wt% of the original polymer) of the indicated second fluoropolymer, andthen the solvent was again removed under an aspirator and then a vacuumpump. The second fluoropolymers were selected from Fluorel FC2145,Fluorel FC2178 and THV-200. Where crosslinking is indicated in Table I,the resin was redisolved in MEK and the indicated amount (as wt % of theoriginal polymer) of Bisphenol AF was added (obtained from Asahi Glass,Tokyo, Japan), and the solvent was again removed under an aspirator andthen a vacuum pump. The crosslinker was activated by heat after thepolymer was cast into a membrane.

[0056] For membranes made using FC2178 as the starting polymer it wasfound that the polymer did not remain in solution but precipitated outupon reaction with the phenoxide. It was found that the product remainedin solution when MEK was replaced with a mixture of solvents composed of1 part (by weight) MEK, 1 part THF, 0.1 part toluene, and 0.25 partmethanol. Membranes Nos. 19-21 in Table I were made and handled usingthis solvent mixture.

[0057] Phenoxy-Substituted Fluoropolymer Films

[0058] The resulting resins were pressed into films in a Carver pressbetween two plates at a temperature of at least 100° C. The resultingfilms were cut into pieces of about 30 to 60 square centimeters.

[0059] Sulfonated Phenoxy-Substituted Fluoropolymer Films

[0060] The resulting film pieces were sulfonated by immersion in an80/20 mixture of concentrated H₂SO₄/fuming H₂SO₄ for the duration andtemperature indicated in Table I, then rinsed until pH stable.

[0061] Ionic Conductivity Measurements

[0062] Conductivity measurements were made according to the followingprocedure, which is based on T. Zawodzinski et. al., J. Phys. Chem.,vol. 95, p. 6040 et seq. (1991). Prior to testing, the membrane samplewas boiled in deionized water for 2 hours. A membrane sample 1 cm wideand at least 2 cm long was clamped at each end by two Pt electrodes, 2cm apart, contacting the membrane surface. The cell was submersed inwater and the ohmic and capacitive components of the membrane impedancewere measured by impedance spectroscopy at a frequency range of from 65kHz to 0.1 Hz using a Solartron frequency analyzer (Solartron, UK). Datawere collected at 25° C. unless otherwise specified. A Nyquist plot,which is a plot of imaginary vs. real impedance, was generated for eachcell. The resulting curve was extrapolated to zero capacitance, whichpoint represents the pure ohmic resistance. Conductivity in S/cm iscalculated from the calculated value of pure ohmic resistance and thecell constant: Conductivity=2/((resistance)×(membrane thickness)).

[0063] Equivalent Weight Measurements

[0064] Membrane samples were weighed and then suspended in about 60 mlof water and titrated with 0.1N NaOH to determine the molar amount ofacid groups (sulfate) in the membrane sample. Equivalent weight (EW) isdetermined by dividing the weight in grams by the amount in moles ofacid groups.

[0065] Oxidative Stability Measurements

[0066] Oxidative stability measurements were made according to thefollowing procedure, which is based on LaConti, Electrochem. Soc. Proc.,Vol. 77-6, p. 354 (1977). Membrane samples were dried under an aspiratorand then a vacuum pump, and weighed. The samples were then placed in a3% hydrogen peroxide solution containing 4 ppm iron sulfate and heatedto 68° C. At measured time intervals, samples were removed, washed withwater, dried under an aspirator and then a vacuum pump, and re-weighed.FIG. 1 shows weight loss for membrane No. 4 of the invention (Trace A)and comparative membranes 22C (Nafion) (Trace B) and 23C (Bio-Rex)(Trace C). While the Bio-Rex membrane (No. 23C) lost over 80% of itsoriginal weight due to oxidative degradation, membrane No. 4 of thepresent invention showed little weight loss and compares well to theNafion membrane (No. 22C).

Example 2

[0067] Preparation and Testing of Li⁺ Single Ion Conducting Membranes Asample of film # 12 from Example 1 was dried in air and then soaked in0.08M LiOH for 30 minutes at room temperature. The sample was thenwashed three times with DI water until the pH of the wash water remainedstable. The film was then dried under vacuum for 17 hours and placed ina glove box. The film was cut in two pieces, Sample A (0.0759 g) wassoaked in a solution of IM BETI (bis-perfluoroethyl sulfonylimide,lithium salt) in 50:50 ethylene carbonate/dimethyl carbonate (EC/DMC),and sample B (0.1524 g) was soaked in 50:50 EC/DMC. After 3 days thefilm samples were removed. Sample A weighed 0.1680 (121% increase) andsample B weighed 0.3683 g (119% increase). Both samples were 0.005 mmthick. Conductivity was measured according to the following method: ForLi ion conducting films the conductivity was measured in a dry box. Thefilm was placed between two circular stainless steel electrodes (5.06cm²) and the conductivity was measured using a Princeton AppliedResearch (PAR) potentiostat/galvanometer model 273 with a Schlumbergermodel 1260 frequency response analyzer. The impedance responses of cellswere measured over a frequency range of 100,000 to 1 Hz. Conductivitywas calculated from the ohmic resistance using the formula:1/s(conductivity S/cm)=1(film thickness in cm)/a(film area incm²)×R(ohms). The measured Li⁺ conductivity for Sample A was 3×10⁻⁴ S/cmand for Sample B was 4×10⁻⁵ S/cm.

[0068] Various modifications and alterations of this invention willbecome apparent to those skilled in the art without departing from thescope and principles of this invention, and it should be understood thatthis invention is not to be unduly limited to the illustrativeembodiments set forth hereinabove. All publications and patents areherein incorporated by reference to the same extent as if eachindividual publication or patent was specifically and individuallyindicated to be incorporated by reference.

We claim:
 1. A method of making an acid functional fluoropolymercomprising the steps of: a) dehydrofluorinating a starting fluoropolymerwith a dehydrofluorinating agent to form an unsaturated fluoropolymer;b) adding an acidifiable nucleophilic functionalizing agent to a doublebond of the unsaturated fluoropolymer to form a fluoropolymer bearing anacidifiable function; and c) acidifying said acidifiable function;wherein said acid functional fluoropolymer is sufficiently acidified asto meet a condition selected from: a) the equivalent weight of thepolymer is 5000 or less; and b) the proton conductivity of the polymerat 25° C. is 0.01 Siemens per centimeter (S/cm) or higher.
 2. The methodof claim 1 wherein the starting fluoropolymer is a polymer or copolymerof vinylidene fluoride (1,1-difluoroethene).
 3. The method of claim 1wherein the starting fluoropolymer and the acid functional fluoropolymerare membranes.
 4. The method of claim 1 wherein said acidifiablenucleophilic functionalizing agent comprises a group selected fromanionic nitrogen-, oxygen- and sulfur-containing groups.
 5. The methodof claim 4 wherein said acidifiable nucleophilic functionalizing agentis an aryloxide (ArO⁻).
 6. The method of claim 5 wherein saidacidifiable nucleophilic functionalizing agent is phenoxide (PhO⁻). 7.The method of claim 1 wherein said acidifiable nucleophilicfunctionalizing agent comprises a group readily hydrolyzable to form anacid functional group.
 8. The method of claim 7 wherein said acidifiablenucleophilic functionalizing agent comprises sulfonyl halide groups. 9.The method of claim 1 wherein said acid functional fluoropolymer has anequivalent weight of 3000 g/mol or less.
 10. The method of claim 1wherein said acid functional fluoropolymer has an equivalent weight of1600 g/mol or less.
 11. An acid functional fluoropolymer according toclaim 1 having an ionic conductivity (H⁺) at 25° C. of 0.01 mS/cm orgreater.
 12. An acid functional fluoropolymer according to claim 1having an ionic conductivity (H⁺) at 25° C. of 0.05 mS/cm or greater.13. The method of claim 1 additionally comprising the step of: d) cationexchange.
 14. The method of claim 13, wherein said cation exchangeyields a acid functional fluoropolymer which is a Li⁺ salt.
 15. Themethod of claim 1 additionally comprising the step of: e) crosslinkingsaid acid functional fluoropolymer.
 16. The method of claim 1additionally comprising the step of: f) mixing said acid functionalfluoropolymer with one or more second fluoropolymers.
 17. An acidfunctional fluoropolymer made according to the method of claim
 1. 18. Apolymeric blend comprising the acid functional fluoropolymer of claim 17and one or more second fluoropolymers.
 19. An ion conducting membrane(ICM) comprising the acid functional fluoropolymer of claim
 17. 20. Amethod of making an ion conducting membrane (ICM) comprising the stepsof: a) dehydrofluorinating a starting fluoropolymer with adehydrofluorinating agent to form an unsaturated fluoropolymer; b)adding an acidifiable nucleophilic functionalizing agent to a doublebond of the unsaturated fluoropolymer to form a fluoropolymer bearing anacidifiable function; c) forming the fluoropolymer bearing anacidifiable function into a membrane; and d) acidifying said acidifiablefunction to form an ICM.
 21. An acid functional fluoropolymer havingpendent groups according to the formula: —X—Ar—A_(n), wherein X isselected from O, S or NR, where R is selected from H and C1-C30 alkyl oraryl, which are optionally substituted, wherein Ar is a C6-C30 aromaticgroup, which is optionally substituted, wherein A is an acidic functionor salt thereof, wherein a can be independently chosen to be 1, 2 or 3;and wherein said acid functional fluoropolymer is sufficiently acidifiedas to meet a condition selected from: a) the equivalent weight of thepolymer is 5000 or less; and b) the proton conductivity of the polymerat 25° C. is 0.01 Siemens per centimeter (S/cm) or higher.
 22. The acidfunctional fluoropolymer according to claim 21 wherein X is O.
 23. Theacid functional fluoropolymer according to claim 21 wherein A is SO₃M,where M is H+ or a metal anion.
 24. The acid functional fluoropolymeraccording to claim 22 wherein A is SO₃M, where M is H+ or a metal anion.25. The acid functional fluoropolymer according to claim 21 wherein Aris phenyl.
 26. The acid functional fluoropolymer according to claim 22wherein Ar is phenyl.
 27. The acid functional fluoropolymer according toclaim 23 wherein Ar is phenyl.
 28. The acid functional fluoropolymeraccording to claim 24 wherein Ar is phenyl.
 29. The acid functionalfluoropolymer according to claim 21 wherein Ar is substituted with oneor more electron donating groups.
 30. The acid functional fluoropolymeraccording to claim 28 wherein Ar is substituted with one or moreelectron donating groups.
 31. An ion conducting membrane (ICM)comprising the acid functional fluoropolymer of claim
 21. 32. An ionconducting membrane (ICM) comprising the acid functional fluoropolymerof claim
 28. 33. A membrane electrode assembly (MEA) comprising the ionconducting membrane (ICM) of claim
 19. 34. A fuel cell comprising themembrane electrode assembly (MEA) of claim
 33. 35. A membrane electrodeassembly (MEA) comprising the ion conducting membrane (ICM) of claim 31.36. A fuel cell comprising the membrane electrode assembly (MEA) ofclaim
 35. 37. A membrane electrode assembly (MEA) comprising the ionconducting membrane (ICM) of claim
 32. 38. A fuel cell comprising themembrane electrode assembly (MEA) of claim
 37. 39. An ion conductingmembrane (ICM) comprising the acid functional fluoropolymer of claim 18.40. A membrane electrode assembly (MEA) comprising the ion conductingmembrane (ICM) of claim
 39. 41. A fuel cell comprising the membraneelectrode assembly (MEA) of claim
 40. 42. A method of making an acidfunctional fluoropolymer comprising the steps of: a) adding anacidifiable function to a fluoropolymer to form a fluoropolymer bearingan acidifiable function; and b) acidifying said acidifiable function;wherein said acid functional fluoropolymer is sufficiently acidified asto meet a condition selected from: a) the equivalent weight of thepolymer is 5000 or less; and b) the proton conductivity of the polymerat 25° C. is 0.01 Siemens per centimeter (S/cm) or higher.
 43. Themethod according to claim 42 wherein said acidifiable function is agroup according to the formula: —X—Ar, wherein X is selected from O, Sor NR, where R is selected from H and C1-C30 alkyl or aryl, which areoptionally substituted, wherein Ar is a C6-C30 aromatic group, which isoptionally substituted.
 44. The method of claim 43 wherein saidacidifiable nucleophilic functionalizing agent is an aryloxide (ArO⁻).45. The method of claim 44 wherein said acidifiable nucleophilicfunctionalizing agent is phenoxide (PhO⁻).
 46. An ion conductingmembrane (ICM) comprising the acid functional fluoropolymer madeaccording to the method of claim
 42. 47. A membrane electrode assembly(MEA) comprising the ion conducting membrane (ICM) of claim
 46. 48. Afuel cell comprising the membrane electrode assembly (MEA) of claim 47.